1. Field of the Invention
The present invention is related to the stacking of material, and more particularly, to a method and mechanism for sensing the position of the top of a stack of paper in a paper stacking apparatus.
2. Background and Material Information
In xerographic and other copiers, scanners, printers or other document imaging systems, it is desirable to automatically deposit either normal "cut sheet" documents or a continuous computer form web document, or "fanfold paper," into a catch tray after imaging, where it is stacked on a paper collection table.
Refolding and stacking of continuous form paper is often accomplished by passive gravity fed stackers, which may take the form of a wire basket or other box shaped configuration. However, problems often arise with these types of stackers, since continuous form papers have a tendency to mis-stack or mis-fold while stacking large numbers of continuous sheets. A mis-stack is defined as any media movement in the stacker that results in a fatal printer fault (i.e., the printer halts), causes the media to overflow the stacker, or requires operator intervention. A mis-fold is defined as abnormal media handling in the stacker that allows printing and stacking to continue and does not result in paper jams, fatal printer faults, or stacker overflows.
With the advent and commonplace use of laser printers, occurrences of mis-stacks and mis-folds have been exacerbated. The heat pressure rollers that the laser printer uses to fuse the toner image onto the printer paper tend to iron out the perforations between the sheets of the continuous form paper. As a result, the paper folds lose their memory and have a tendency not to easily refold into a neat stack. As printer speeds have improved over time, the occurrences of mis-folds and mis-stacks have further increased.
To reduce the likelihood of such mis-folds and mis-stacks and increase the stacking reliability, it has become commonplace to maintain a relative constant geometry between the collection area of the stacker and the output area of the imaging device. Related art active stacking apparatuses utilize spring-type paper collection tables that must be counterbalanced. This type of apparatus is unreliable in that paper length, widths and thicknesses may vary, resulting in varied weight. The paper collection table may therefore lower too great or too small of an amount, resulting in mis-folds and mis-stacks.
U.S. Pat. No. 4,460,350, which issued to Mittal et al. on Jul. 17, 1984, and is expressly incorporated herein by reference in its entirety, discusses a continuous paper stacking device. The device of this invention includes LEDs and optical detectors that enable a paper collection tray to be lowered when light from the LEDs is blocked by a paper stack.
Other related art devices employ active stacking apparatus having paper tables that descend in accordance with a longitudinal amount of paper printed. One such device is discussed in U.S. Pat. No. 4,508,527 which issued to Uno et al. on Apr. 2, 1985. This device has several shortcomings, given that papers have varied dimensions, requiring the lowering of the paper table at differing intervals.
U.S. Pat. No. 4,718,807, which issued to Baxter on Jun. 12, 1988, discusses a signature stacking apparatus for the stacking of signatures. The signatures are fed onto the stack, which is supported by a support arm, and sprockets are rotated to move a chain to maintain the upper surface of the stack constant. As signatures are fed onto the stack, a sensor detects the rising level of the stack and effects operation of controls to cause a motor to rotate the sprockets, thereby lowering the support arm.
Copending application Ser. No. 08/907,615, which is expressly incorporated herein by reference in its entirety, describes an apparatus which actively refolds and stacks continuous form paper. The apparatus has a plurality of rotating paddles which assist in packing down paper as it is placed on a paper collection table.
Having a paper stacking apparatus that has none of the above-mentioned problems is therefore desirable.